Experimental and analytic modeling of piercement structures

Piercement structures such as mud volcanoes, hydrothermal vent complexes, pockmarks and kimberlite pipes, form during the release of pressurized fluids. The goal of this work is to predict under which conditions piercement structures form from the insights gained by sand box experiments injecting compressed air through an inlet of width w at the base of a bed of glass beads of height h. At an imposed critical velocity v(f), a fluidized zone consisting of a diverging cone-like structure formed with morphological similarities to those observed in nature. Dimensional analysis showed that v(f) is correlated to the ratio of h over w. In addition, we derived an analytical model for v(f) which is compared to the experimental data. The model consists of a force balance between the weight and the seepage forces imparted to the bed by the flowing gas. The analytic model reproduces the observed correlation between v(f) and h/w, although a slight underestimate was obtained. The results suggest that the gas-particle seepage force is the main triggering factor for fluidization and that the commonly used proxy, which the fluid pressure must equal or exceed the lithostatic weight, needs to be reconsidered. By combining the experiments and the model, we derived critical pressure estimates which were employed to a variety of geological environments. Comparing the estimated and measured pressures prior to the Lusi mud volcano shows that the presented model overestimates the critical pressures. The model paves the way for further investigations of the critical conditions for fluidization in Earth systems.